Nix Flakes

If you’re using flakes, you can use the following minimal flake.nix as an example:

{
  description = "NixOS configuration";

  inputs.simple-nixos-mailserver.url = "gitlab:simple-nixos-mailserver/nixos-mailserver/nixos-20.09";

  outputs = { self, nixpkgs, simple-nixos-mailserver }: {
    nixosConfigurations = {
      hostname = nixpkgs.lib.nixosSystem {
        system = "x86_64-linux";
        modules = [
          simple-nixos-mailserver.nixosModule
          {
            mailserver = {
              enable = true;
              # ...
            };
          }
        ];
      };
    };
  };
}